Anti-β₂GPI antibodies stimulate phosphorylation of myosin RLC. (A) Phosphorylation of RLC residues T18 and S19 in response to anti-β₂GPI antibodies. Total lysates prepared from endothelial cells incubated with β₂GPI and anti-β₂GPI antibodies for 60 minutes were subjected to immunoblotting using an anti–phospho-RLC (T18/S19) antibody. After blotting with the phosphorylation-specific antibody, blots were stripped and reblotted with an antibody to RLC. (B) The same lysates depicted in panel A were immunoblotted using an antibody specific for phosphorylated S19 in RLC. (C) Densitometric analysis of panel A using NIH ImageJ. (D) Densitometric analysis of panel B using NIH ImageJ.